The present innovation is related to the optical telecommunication, Particularly to the design and manufacture of tunable optical devices for WDM (Wavelength Division Multiplexed) systems. WDM technology highly expends the capability of modem optical communication network. Tens of wavelengths located between 1530 to 1570 mn can pass through a signal fiber by this technology. Each wavelength corresponds to a Channel. With the development of the optical telecommunication, the network becomes more and more complicated in structures. The network should be intelligent, meaning be more flexible and scalable. These characteristics strongly depend on tunable devices, such as optical switches, variable optical attenuators (VOA) and tunable filters.
Optical switches are used to selectively reroute signals or to control the traveling direction of light beams. VOA are used to adjust light intensities so the network could work in order. Tunable filters are used to select a certain wavelength among many wavelengths and the network can be dynamically managed.
These devices are basic in optical network, and they should have the characteristics such as small size, low power consumption, high reliability, high tuning speed and small insertion loss. Until now, switches, VOA and tunable filters can be made based on different principles, such as mechanical moving, magneto optical effect, electro optical effect, acoustic optical effect and thermal effect. For devices based on magneto optical effect, the power consumption is high. And due to the magnetic parts, they are hard to be made small. For devices based on electro optical effect, the applied voltage is usually quite high. For devices based on acoustic optical effect, side lobes are strong and insertion loss is large. For devices based on thermal effect, the tunable range is small and the response time is usually long.
As for the mechanical principle, it can be divided in several methods. Some devices adopt motors to drive optical parts, some adopt piezocrystal. All these methods need heavy labor on assembly. Recently, MEMS technology has been introduced into manufacturing optical devices. It adopts the state-of-the-art technology of semiconductor to batch fabricate small-sized devices.
For MEMS technology, reflection by micro-mirrors can be adopt to make VOA and switches, while the robust of micro-mirrors is poor; Diffraction by grating is only good for making VOA; Interference devices based on the Fabry-Perot cavity can have very narrow resonant frequency, and is okay to be used for monitoring channels. However, it is poor in re-routing channels. Every optical channel has bandwidth due to modulation, and some information of channels will be filtered out and the error rate will increase if we use the Fabry-Perot cavity as tunable filters to select channels. This shortcoming will become severe in DWDM. Interference devices based on the Fabry-Perot cavity have poor performance for VOA and switches since the wavelength dependent loss (WDL) is high.
Fabry-Perot resonators with multiple-cavity can have flat passband with high resolution. Tunable filters having this structure will be suitable for re-routing channels. When the reflectivity of the resonators is low, this structure can also be adopted for VOA and Switches. Since this structure requires substantially identity of cavities, Macro devices based on this structure is difficult to be made and lack mechanical stability. U.S. Pat. No. 6,424,466 give a related MEMS-based design of tunable filters, but it is mainly on the process of assembling.